Significant diminishment of surface smoothness in articles prepared by extrusion of thermoplastic resins is generally referred to as "melt fracture."
The tendency of resins to melt fracture has been the subject of considerable past research efforts. Prior conventional approaches to this problem typically involve modifications to the resin in order to reduce the sensitivity of the material to melt fracture. Resin modifications, such as broadening the molecular weight distribution, however, tend to reduce the physical properties of the resin below that which can be obtained from the unmodified resin.
Accordingly, it is an object of this invention to provide improved processes and apparatus for extruding synthetic resinous thermoplastic materials under conditions permitting a broader latitude in the selection of resin with regard to the problem of melt fracture, i.e., processes and apparatus which operate in a manner to reduce the sensitivity of various resins to melt fracture.
It is a further objective of this invention to provide such processes and apparatus which beneficially allow the rate of extrusion to be increased while maintaining or improving the smoothness of the surface finish of the extruded product.